1. Field of the Invention
This invention relates to a surface acoustic wave device wherein a surface acoustic wave element and a matching circuit therefor are formed on a single substrate.
2. Description of the Prior Art
A surface acoustic wave device is made of a piezoelectric single crystalline material such as crystal, lithium niobate (LiNbO.sub.3), etc., a piezoelectric ceramic material, or alternatively a combination of a non-piezoelectric plate and a piezoelectric film deposited thereon. To actually use the surface acoustic wave device as a circuit constituent, it is necessary to match the device with outer or external circuits.
FIG. 1 shows a conventional construction to use such a surface acoustic wave device as a circuit constituent. The surface acoustic wave device 4 includes a piezolelectric substrate 1, an input transducer 2 and an output transducer 3 both provided on the substrate 1. Matching circuits 5, 6 are each connected at one end thereof to a respective one of the input and output ends of the device 4 and are connected at the other ends to a signal source 7 and to a load 8, respectively. When an electric signal is applied to the input transducer 2 from the signal source 7 via the matching circuit 5, it is converted into a surface acoustic wave by the input transducer 2. The surface acoustic wave travels along the surface of the piezoelectric substrate 1, and when it reaches the output transducer 3, it is reconverted to an electric signal. The electric signal is then applied to the load 8 via the matching circuit 6. In this process, the matching circuits 5 and 6 are indispensable to reduce conversion loss caused upon conversions from an electric signal to a surface acoustic wave or vice versa. So, almost all of such devices are accompanied by such matching circuits at least at the input ends or the output ends thereof.
The matching circuits 5 and 6 each consist of an independent member such as an inductive equivalent element (a coil, for example), a capacitive equivalent element (a condenser, for example), etc. These elements are bulky as compared to the surface acoustic wave device itself. As the result, a considerably large space is needed as a whole for provision of the surface acoustic wave device for actual use thereof as a circuit constituent. Further, the number of the parts of the device and its mounting processes increase because the coils and the condensers are independent members. Additionally, the matching circuits must be adjusted after being assembled. These circumstances cause an increase in the production cost of the device.